1. Field of the Invention
The present invention relates to electric energy measurements for electric utility systems, and more particularly such measurements utilizing digital processing and calculations by programmed control of solid state circuits.
2. Description of the Prior Art
In the field of electric utility power and energy measurements, electromechanical induction type watthour meters have been the most extensive type in commercial use. Although many electronic measuring circuits are known, improvements in their accuracy, ruggedness, reliability and cost is desired.
Analog system multiplying circuit techniques have been chiefly used in the electronic power and electric energy measuring circuits to compute the product of the voltage and current components of an electric energy quantity to be measured. Power consumption or the electric energy usage in kilowatt-hours and kilowatt demand is required for billing purposes by an electric utility company and a computed power quantity must be calculated by the measurement circuits by deriving the product of voltage and current. The power quantity must be integrated with respect to time to produce kilowatt-hour measurements. It is further desirable to have circuits with the capability of measuring additional electric energy parameters of a power line system, such as reactive kilovolt-ampere hours, volts-squared hours and ampere-squared hours for load studies.
In one solid state measuring circuit described in U.S. Pat. No. 3,764,908 issued Oct. 9, 1973 and assigned to the assignee of this invention, voltage and current related signals are both applied to a semiconductor device having a logarithmic computing characteristic. Accordingly, an output signal is produced which is equal to the product of the input signals.
Another known analog multiplier technique includes time division multiplication. For example, a voltage signal is sampled to derive a pulse width modulated signal corresponding to the voltage amplitudes. The current signal is sampled at a rate controlled by the variable pulse width signal. An output is produced consisting of a series of pulses having a height proportional to the instantaneous current values and pulse width proportional to the instantaneous voltage values. The resultant signal is filtered to obtain an average value of the pulses which is, in turn, proportional to the instantaneous power. The average value signal is applied to a voltage-to-frequency converter utilizing integrating capacitors, for example. Variable frequency pulses of the converter are applied to a magnetic recorder or pulse totalizer formed by an electromechanical counter or electronic counting circuit. The accumulated pulses are representative of the electric energy kilowatt-hour consumption for billing or power analysis by an electric utility company. Electronic circuits for measuring different electric energy parameters are disclosed in U.S. Pat. Nos. 3,864,631, issued Feb. 4, 1975, and 3,778,794, issued Dec. 11, 1973, both assigned to the assignee of this invention.
It has been found that the analog electronic circuit techniques are sometimes difficult to apply in order to obtain the desired accuracies. Accurate drift-free analog multipliers are often expensive and it is further difficult to obtain square root computing circuits in the analog circuit field which are sometimes required for calculating electric power quantities. Also, analog integration circuits required in the analog electronic power measuring apparatus produce undesired drift and variations over long time intervals. In time division multiplication circuits it is known that frequency dependent sampling occurs at the multiplier with the associated digital integration also having similar dependency upon variations of integrating capacitors.
A more ideal approach to electric power measurements is to utilize digital processing techniques wherein voltage and current signals are sampled at very high rates, for example in the order of 1,000 times per cycle. The instantaneous sampled values would be quantized in high resolution analog-to-digital converters having a high order of bits provided in the binary representations of the digitizing outputs thereof. The time for the amplitude quantizations would be negligible and the speed of the digital processor circuitry would be sufficiently fast so that all of the calculations would keep up with the fast sampling rates.
The known advantages of an ideal digital approach, after the analog values have been converted to digital signals, is that there are no changes of accuracy occurring due to component drift and variations. Calculation of different electric power and energy parameters are more easily accomplished in digital circuits. The use of programmable read only memories permit flexibility in the measuring apparatus so that it is possible to accomplish many different functions without substantial changes in hardware.
The disadvantages of an ideal digital measuring apparatus include the use of analog-to-digital converters having very high speeds and high resolution which are rather expensive since these two characteristics are competing from a design standpoint due to the fact that more time is typically required to achieve higher resolutions in the quantizations. The higher resolution outputs of such converters produce higher order binary word lengths to represent the analog values so that that associated circuits become more complex and expensive. Further, digital processing circuits capable of operating at varying fast speeds are substantially more complex, expensive and require higher operating power supplies.
Alternatively, a digital processing power measuring apparatus capable of operating at slower sampling speeds and digitizing the instantaneous signal values with less resolution retains the advantages of the digital techniques including stability and flexibility at substantially lower cost. The reduction in resolution permits handling of shorter binary words with less bits for producing the digitized binary representations of the analog signal amplitudes. Slower sampling speeds permit digital processing at lower speeds to simplify the digital processing circuitry. However, the reduced sampling rates and lower digitizing resolution have a corresponding reduction in the accuracy of the digital representations of the sample amplitudes and a reduction in the true digital representation of each complete cycle of the input analog signals.
Examples of prior art patents generally disclosing the use of digital circuit techniques and digital calculations in power line and power measuring systems include U.S. Pat. No. 3,758,763, issued Sept. 11, 1973, disclosing a method of digitally processing AC signals utilizing a digital computer. Input AC power signal components are sampled and digitized at predetermined sampling times. A first sampled value is stored and then a second sampled value is obtained and the product of the two signals is derived to determine if the product has a negative sign, indicating that a zero crossing has occurred in the sampled signal. The disclosed system obtains frequencies, phase differences, powers and impedances of a power line network rather than measurement of electric energy parameters as in the present invention.
In U.S. Pat. No. 3,569,785, issued Mar. 9, 1971, a computer control system is disclosed for power system protective relaying, rather than for electric power measurement, in which the voltage and current components of an AC power line network are sampled and digitized and then applied to a computer for calculation of relaying control functions.
In measuring R.F. power, U.S. Pat. No. 4,011,509 issued Mar. 8, 1977, describes a digital measuring circuit that stores one digital power value and compares it to a measured power value to obtain a relative power value for expressing the measurement in decibels, not used in the subject invention.
Examples of electromechanical induction type watthour meter based metering packages having electric energy measurements produced that are closely related to the measurements produced in the present invention is the magnetic tape metering package designated as types WRS, WRR and WRP. The aforementioned types of metering packages are available from the Westinghouse Electric Corporation, Meter and Instrument Transformer Division, Raleigh, North Carolina. The metering packages include a load survey type magnetic recorder, and a combination of a polyphase watthour meter, a Q-hour meter, and a V.sup.2 -hour or A.sup.2 -hour meter wherein each meter is equipped with an electronic pulse initiator for producing output pulses responsive to rotation of the meter movements. The metering packages are designed to monitor a variety of typical electric utility services for load research or billing applications. The electric energy parameters that can be obtained from the metering package data outputs include kilowatt-hours, kilowatt-demand, reactive KVA hours, volts-squared hours or ampere-squared hours to provide a complete analysis of the electric energy quantity measured and electric load profile for a given metering installation. The data can be used for load research or cost of service data, or for operational data relating to feeder loading, billing equipment sizing and other uses by an electric utility company.
Although the aforementioned metering packages are satisfactory in many applications, they are bulky, subject to limitations in the flexibility of the outputs and different energy measurements obtainable because of the use of a plurality of induction type meter devices.
Accordingly, it is desirable to utilize the optimum advantages of digital processing and calculating techniques for AC electric energy measuring by optimum use of digital circuit arrangements operating at optimum signal processing rates while obtaining desired reliability and accuracy in the measured and calculated electric energy parameters.